1. Field of the Invention
The present invention relates to a detecting apparatus for detecting a magnetic minute substance. More specifically, the detecting apparatus of the present invention can detect continually at a high sensitivity an entity of fine particles, without destroying an object to be inspected having the shape of a wire or a rod.
2. Description of the Related Art
Foreign matter contained in a long material is a possible cause of serious trouble, even if the foreign matter is very small. For example, Cu is a wire material used as an electric conductor. Some times, it is thinned to less than 100 .mu.m in diameter. However, Cu runs out on an elongated process, when a foreign matter of less than 10 .mu.m is contained in the Cu material. As another example, when foreign matter of less than 10 .mu.m exists in its isolation coating, proof pressure of a cable line conveying power at a high voltage of 500 kV significantly deteriorates.
A method for detecting foreign matter in a material using electromagnetic induction is known. However, this method only can detect foreign matter more than approximately 100 .mu.m. Accordingly, small foreign matter less than 100 .mu.m cannot be detected. A new method that can detect smaller foreign matter is desired.
For this technical subject, the applicant has filed a patent application relating to a new detecting apparatus, namely U.S. patent application Ser. No. 08/273,433.
This apparatus detects a drift of a magnetic field that includes an object to be inspected caused by the existence of foreign matter. This equipment comprises a SQUID (Superconducting Quantum Interference Device) as a magnetic sensor having very high sensitivity. Accordingly, the output signal of the SQUID is affected by a drift of the environmental magnetic field. The circumference of the SQUID and the object to be inspected is surrounded with a magnetic shield container, and detection of the magnetic substance is done inside of the magnetic shield container. The magnetic shield container may be made of a permalloy for example.
However, when an object to be inspected is a very long object, like a wire, the object to be inspected cannot be accommodated in a magnetic shield container. Then, the detection is enforced while the object to be inspected is carried in and out of the magnetic shield container. In other words, during detection, the object to be inspected is introduced at one end of the magnetic shield container and carried out from the other end of the magnetic shield container simultaneously. Accordingly, a pair of openings needs to be made in the magnetic shield container. The object to be inspected is carried in and out through these openings.
However, immersion of outside magnetic field must be prevented from invading the magnetic shield container through the openings. In the patent application, a magnetic shield container having a special shape is used for inspecting a long object to be inspected and immersion of the outside magnetic field is prevented by the shape of the container.
But, it is difficult to produce a magnetic shield container having a complicated shape. At the same time, it is difficult to handle the container because the complicated shape complicates the operation of inserting a long material.